The crystal is attackable when Nex reaches 600,000 life points. It can only be attacked depending on the order of the praesul (Nex's mages) were killed. Unlike the other crystals, this will not allow Nex to use attacks but will heal her over time.

^The Promised Gift, written by Zaros, RuneScape. "The stone of blood was once the bloody matriarch of Vampyrium. This trueborn vampyre was the mother of my once-servant Drakan, and during my time on Gielinor had fallen from my loyal service and grown corpulent in the manner of ancient Hostilius."